In heating food, the food begins to boil when the cooking conditions of the food undergo a conspicuous change in temperature at about 92.degree. to 97.degree. C. The length of time from the start of heating to the boiling point of the food depends on the type of food, involving such factors as the specific heat or microwave absorption capacity of the food or the amount of the food and such other factors as the size or the number of units of the food. In addition, such a length of time is greatly affected by the initial temperature of the food.
Conventional cooking ovens using the detection of humidity are known, for example, from U.S. Pat. Nos. 3,839,616 and 4,097,707. One typical conventional cooking oven operates in the manner shown in FIG. 1. The time length .tau..sub.o from the start of heating the food to the start of boiling of the food is measured and the food further continues to be heated until it is cooked, after a time length K.tau..sub.o, i.e., the product of the time length .tau..sub.o and a constant K specific to the food determined by the type and the finished condition of the food. The amount of heat Q required for the food to be properly cooked may be expressed by the equation shown below. EQU Q=m.multidot.C.multidot.(Tf-Ti)+m.multidot.B (1)
where C is the specific heat of the food, m the quantity of the food, Tf the temperature at which the water in the food begins to boil, and B a value specific to the food, the value being dependent on the amount of heat associated with the change in quality of the food (or degeneration) and the latent heat of evaporation.
The value Q is given as the product of the microwave power p and the total heating time of the food .tau., namely, Q=.rho..tau.. Thus the total heating time of the food is expressed as ##EQU1## The first term of the right side of this equation represents the time length from the starting of heating the food to the boiling of the water in the food, and the second term thereof is considered to represent the time from the start of boiling of the food to the properly cooked up state of the food. Therefore, the equation (2) may be rewritten as ##EQU2##
In the conventional cooking oven using the detection of humidity, the initial temperature of the food T.sub.i in equation (3) above is assumed to be about 20.degree. C. so that K is considered for convenience to be a constant, with the result that the food heating time is given by equation (3).
The food to be heated for cooking, however, includes that which has been left to stand at a high room temperature, that just taken out of the refrigerator, and frozen foods, etc., which vary greatly in their initial temperature immediately before being heated. The value K for such a wide range of food is not constant but varies greatly. In conventional cooking ovens wherein the value K is assumed to be constant, some types of food or food under certain conditions fail to be finished satisfactorily by reason of the great variety in the initial temperature of the food to be cooked and the resulting wrong setting for the total heating time.
An object of the present invention is to provide a method of food heating control in which the required energy is accurately calculated in spite of different amounts, materials or shapes including the thickness of the food to be cooked, thus realizing automatic cooking in all cooking processes only by designating the type of food without being greatly affected by the amount, shape or initial temperature of the food.